📚 A-Level Edexcel Biology: Last-Minute Revision Notes | A-Level Edexcel 生物：考前冲刺笔记
Welcome to your concise, last-minute revision notes for Edexcel A-Level Biology. These notes highlight the essential concepts, keywords, and equations and practical skills you need to know for Papers 1, 2 and 3. Use them to refresh your memory, check definitions and build confidence in the final days before the exam.
欢迎阅读 Edexcel A-Level 生物考前简明冲刺笔记。本文梳理 Paper 1、Paper 2 与 Paper 3 必备的核心概念、关键词、方程式与实验技能，帮助你在考前最后几天快速回顾、核对定义并增强信心。
Each section below pairs the English explanation with a Chinese translation, so you can revise bilingually and check that you can express every idea in precise scientific language. Exam marks are usually awarded for keywords, so read the bold terms carefully.
以下每一节都将英文讲解与中文翻译配对呈现，方便你双语复习，并确认自己能用精确的科学语言表达每个概念。考试得分点通常落在关键词上，请特别留意加粗术语。
1. Monomers, Polymers and the Importance of Water | 单体、多聚体与水的重要性
Monomers are small repeating units that join together to form polymers through condensation reactions. Each condensation reaction forms a covalent bond and releases one molecule of water. The reverse process, hydrolysis, adds water to break the bond and separate the monomers again.
单体是重复的小单位，通过缩合反应连接形成多聚体。每一次缩合反应都会形成一条共价键并释放一分子水。逆转过程为水解，即加水断键，使单体重新分开。
The four main biological polymers and their bonds are summarised below. Knowing the bond name is a common one-mark answer in Edexcel papers.
四种主要的生物多聚体及其连接键总结如下。准确写出键的名称，往往是 Edexcel 题目中一分的直接答案。
- Monosaccharides to polysaccharides - glycosidic bonds | 单糖连接为多糖 - 糖苷键
- Amino acids to polypeptides - peptide bonds | 氨基酸连接为多肽 - 肽键
- Glycerol and fatty acids to triglycerides - ester bonds | 甘油与脂肪酸连接为甘油三酯 - 酯键
- Nucleotides to DNA and RNA - phosphodiester bonds | 核苷酸连接为 DNA 与 RNA - 磷酸二酯键
Water is essential because of its unusual properties. It has a high specific heat capacity, so it buffers temperature changes. It has a high latent heat of vaporisation, making sweating an effective cooling mechanism. It is an excellent solvent for ions and polar molecules, it is cohesive so it forms continuous columns in the xylem, and it is a metabolite in hydrolysis and condensation reactions.
水的关键性质决定了它的重要性：比热容高，可缓冲温度变化；汽化潜热高，因此出汗是有效的降温方式；对离子和极性分子是优良溶剂；具有内聚力，因此在木质部中形成连续水柱；同时它还是水解与缩合反应的代谢物。
2. Carbohydrates: Structure, Tests and Function | 碳水化合物：结构、检测与功能
Carbohydrates contain carbon, hydrogen and oxygen. Monosaccharides such as glucose, fructose and galactose are single sugar units. Two monosaccharides join by condensation to form a disaccharide: glucose plus glucose gives maltose, glucose plus fructose gives sucrose, and glucose plus galactose gives lactose.
碳水化合物由碳、氢、氧组成。单糖如葡萄糖、果糖和半乳糖是单个糖单位。两个单糖经缩合形成双糖：葡萄糖加葡萄糖为麦芽糖，葡萄糖加果糖为蔗糖，葡萄糖加半乳糖为乳糖。
You must know the food tests. Benedict s test detects reducing sugars and turns from blue to brick-red on heating; all monosaccharides and most disaccharides except sucrose are reducing. Iodine solution detects starch and turns from yellow-brown to blue-black. Sucrose must first be hydrolysed with dilute acid and neutralised before it gives a positive Benedict s result.
你必须掌握食物检测方法。本尼迪克特试剂检测还原糖，加热后由蓝色变为砖红色沉淀；除蔗糖外，所有单糖和大多数双糖都是还原糖。碘液检测淀粉，由黄褐色变为蓝黑色。蔗糖必须先用稀酸水解并中和后，才能得到本尼迪克特试剂阳性结果。
Starch, glycogen and cellulose are all polymers of glucose but have different functions. Starch is a mixture of amylose, which is unbranched and coiled, and amylopectin, which is branched. It is the storage polysaccharide of plants, insoluble, osmotically inactive and compact. Glycogen is highly branched, so it is rapidly hydrolysed to release glucose for respiration in animals. Cellulose has straight unbranched chains of β-glucose linked by β-1,4 glycosidic bonds, and hydrogen bonds between chains form microfibrils that give plant cell walls strength.
淀粉、糖原和纤维素都是葡萄糖的多聚体，但功能不同。淀粉由无分支且卷曲的直链淀粉和带分支的支链淀粉组成，是植物的储存多糖，不溶、不产生渗透效应且结构紧凑。糖原高度分支，可被快速水解为动物呼吸作用提供葡萄糖。纤维素由 β-葡萄糖通过 β-1,4 糖苷键形成直链无分支结构，链间氢键形成微纤维，赋予植物细胞壁强度。
3. Lipids: Triglycerides, Phospholipids and Cholesterol | 脂质：甘油三酯、磷脂与胆固醇
Triglycerides are composed of one glycerol molecule and three fatty acids, joined by ester bonds formed in condensation reactions. They are energy-storage molecules, hydrophobic, and less dense than water, which is why fat provides buoyancy and insulation in aquatic mammals. Oxidation of fat releases more energy per gram than carbohydrate, and the oxidation also produces metabolic water.
甘油三酯由一个甘油分子和三个脂肪酸通过缩合形成的酯键连接而成。它们是储能分子，疏水且密度小于水，因此脂肪能为水生哺乳动物提供浮力和保温。每克脂肪氧化释放的能量高于碳水化合物，氧化过程还会产生代谢水。
Phospholipids have two fatty acid tails and one phosphate group, so one end is hydrophobic and the other is hydrophilic. This amphipathic nature means that in water they form a bilayer, with tails pointing inwards and heads outwards. This is the basic structure of all cell membranes, and it creates a hydrophobic barrier to water-soluble substances.
磷脂含有两条脂肪酸尾和一个磷酸基团，因此一端疏水、一端亲水。这种两亲性质使它们在水中形成双分子层，尾部相对向内、头部向外。这是所有细胞膜的基本结构，并形成了对水溶性物质的疏水屏障。
Cholesterol is a lipid that fits between phospholipid molecules in animal cell membranes. It regulates membrane fluidity and stability: it restricts movement of phospholipids at high temperatures and prevents the membrane from becoming too rigid at low temperatures. It is also the precursor of steroid hormones such as testosterone and oestrogen. The emulsion test confirms the presence of lipid: ethanol is added, then water, producing a white emulsion.
胆固醇是一种插入动物细胞膜磷脂分子之间的脂质，可调节膜的流动性与稳定性：高温时限制磷脂移动，低温时防止膜过度刚性。它也是睾酮和雌激素等类固醇激素的前体。乳化测试可确认脂质存在：先加乙醇，再加水，形成白色乳浊液。
4. Proteins and Enzymes | 蛋白质与酶
Amino acids are the monomers of proteins. Each amino acid has a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom and a variable R group. Condensation between the amino group of one amino acid and the carboxyl group of another forms a peptide bond, producing a dipeptide and then a polypeptide.
氨基酸是蛋白质的单体。每个氨基酸的中心碳原子连接一个氨基、一个羧基、一个氢原子和一个可变 R 基团。一个氨基酸的氨基与另一个氨基酸的羧基发生缩合，形成肽键，先生成二肽，再生成多肽。
Protein structure has four levels. The primary structure is the sequence of amino acids, determined by DNA. The secondary structure is alpha helices and beta pleated sheets held by hydrogen bonds. The tertiary structure is the overall 3D shape, held by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions. The quaternary structure involves more than one polypeptide chain, as in haemoglobin. The shape of a protein determines its function, so denaturation by heat or extreme pH destroys function.
蛋白质结构分四级。一级结构是氨基酸序列，由 DNA 决定。二级结构是 α 螺旋与 β 折叠，由氢键维持。三级结构是整体三维形状，由氢键、离子键、二硫键和疏水相互作用维持。四级结构涉及多条多肽链，如血红蛋白。蛋白质的形状决定其功能，因此高温或极端 pH 导致的变性会破坏其功能。
Enzymes are biological catalysts that lower the activation energy of a reaction. They are globular proteins with an active site complementary in shape to a specific substrate, explained by the lock-and-key model and the more accurate induced-fit model. Turnover rate is affected by temperature, pH, substrate concentration and enzyme concentration. Competitive inhibitors bind to the active site, while non-competitive inhibitors bind elsewhere and change the shape of the active site; both reduce the rate, but only non-competitive inhibition cannot be overcome by increasing substrate concentration.
酶是降低反应活化能的生物催化剂。它们是球状蛋白，活性位点形状与特定底物互补，可用锁钥模型以及更准确的诱导契合模型解释。反应速率受温度、pH、底物浓度和酶浓度影响。竞争性抑制剂结合活性位点，非竞争性抑制剂结合其他部位并改变活性位点形状；两者都会降低速率，但只有非竞争性抑制不能通过增加底物浓度来克服。
5. Cell Structure and Membranes | 细胞结构与细胞膜
Eukaryotic cells contain membrane-bound organelles. The nucleus contains DNA and controls protein synthesis through mRNA. Mitochondria are the site of aerobic respiration, with a folded inner membrane called cristae that increases the surface area for the electron transport chain. Ribosomes build polypeptides. The rough endoplasmic reticulum folds and transports proteins, the Golgi apparatus modifies and packages them into vesicles, and lysosomes contain hydrolytic enzymes.
真核细胞含具膜细胞器。细胞核含 DNA，通过 mRNA 控制蛋白质合成。线粒体是有氧呼吸的场所，内膜折叠形成嵴，增大电子传递链的表面积。核糖体合成多肽。粗面内质网折叠并运输蛋白质，高尔基体修饰并将其包装成囊泡，溶酶体含有水解酶。
Prokaryotic cells such as bacteria are much smaller and have no membrane-bound organelles. They have circular DNA not associated with histones, smaller 70S ribosomes, a cell wall made of peptidoglycan, a capsule and sometimes plasmids, which are small circular DNA molecules that can carry antibiotic-resistance genes.
细菌等原核细胞体积小得多，且没有具膜细胞器。它们具有不与组蛋白结合的环状 DNA、较小的 70S 核糖体、由肽聚糖组成的细胞壁、荚膜，有时还有质粒，即小型环状 DNA 分子，可携带抗生素抗性基因。
olesterol in animal cells. The hydrophobic interior restricts the movement of polar and charged substances, so transport proteins are needed. Glycoproteins and glycolipids act as receptors and antigens for cell recognition. Magnification can be calculated using: magnification = image size / actual size, and you must be confident converting between mm, μm and nm.
流动镶嵌模型把细胞膜描述为由磷脂双分子层构成，其中镶嵌蛋白质、糖蛋白和糖脂，动物细胞中还有胆固醇。疏水的内部结构限制极性物质和带电物质的通过，因此需要转运蛋白。糖蛋白与糖脂作为受体和抗原，参与细胞识别。放大倍数可用下式计算：放大倍数 = 像的大小 / 实际大小，并且你必须熟练换算 mm、μm 与 nm。
6. Transport Across Membranes | 跨膜运输
Simple diffusion is the passive movement of small non-polar molecules such as oxygen and carbon dioxide down a concentration gradient. Facilitated diffusion uses channel or carrier proteins for polar molecules and ions such as glucose and sodium ions, and is also passive. Both stop when equilibrium is reached and are accelerated by a steeper gradient, a shorter diffusion distance and a larger surface area.
简单扩散是小分子非极性物质如氧气和二氧化碳顺浓度梯度进行的被动运输。易化扩散借助通道蛋白或载体蛋白运输葡萄糖、钠离子等极性分子和离子，同样属于被动运输。两者在达到平衡时停止，并因更陡的浓度梯度、更短的扩散距离和更大的表面积而加快。
Osmosis is the movement of water from a region of higher water potential to a region of lower water potential through a partially permeable membrane. Water potential is measured in kPa and pure water has a value of zero. Solutes lower water potential, so the water potential of a cell is calculated as:
渗透作用是水分子通过部分透性膜，从水势较高处向水势较低处的移动。水势单位为 kPa，纯水的水势为零。溶质降低水势，因此细胞的水势可由下式计算：
Ψ = Ψₛ + Ψₚ
where Ψₛ is the solute potential and Ψₚ is the pressure potential. Animal cells burst in pure water while plant cells become turgid because the cellulose cell wall exerts pressure that resists further water entry. Active transport uses ATP and carrier proteins to move substances against a concentration gradient, and it can therefore take up ions from very dilute solutions, as in root hair cells. Bulk transport involves endocytosis and exocytosis.
其中 Ψₛ 为溶质势，Ψₚ 为压力势。动物细胞在纯水中会破裂，而植物细胞因纤维素细胞壁产生压力、阻止水分继续进入而变硬挺。主动运输利用 ATP 和载体蛋白逆浓度梯度运输物质，因此根部毛细胞能从极稀溶液中吸收离子。大分子运输则依靠胞吞与胞吐。
7. DNA, RNA and Protein Synthesis | DNA、RNA 与蛋白质合成
Nucleotides have a phosphate group, a pentose sugar and a nitrogenous base. DNA is a double helix of two antiparallel polynucleotide strands held by hydrogen bonds between complementary base pairs: adenine pairs with thymine by two hydrogen bonds, and cytosine pairs with guanine by three. The sugar-phosphate backbone is held by phosphodiester bonds. RNA is single-stranded, contains ribose instead of deoxyribose and uracil instead of thymine.
核苷酸由磷酸基团、五碳糖和含氮碱基组成。DNA 是两条反向平行多核苷酸链构成的双螺旋，链间由互补碱基对之间的氢键连接：腺嘌呤与胸腺嘧啶之间形成两个氢键，胞嘧啶与鸟嘌呤之间形成三个。糖-磷酸骨架由磷酸二酯键连接。RNA 为单链，含核糖而非脱氧核糖，含尿嘧啶而非胸腺嘧啶。
DNA replication is semi-conservative. Helicase separates the strands, hydrogen bonds break, and each strand acts as a template. Free nucleotides line up by complementary base pairing and DNA polymerase catalyses the formation of phosphodiester bonds. Each new DNA molecule contains one original strand and one new strand, which Meselson and Stahl demonstrated using nitrogen isotopes.
DNA 复制是半保留复制。解旋酶将两条链分开，氢键断裂，每条链作为模板。游离核苷酸通过互补碱基配对排列，DNA 聚合酶催化形成磷酸二酯键。每个新 DNA 分子含一条原链和一条新链，Meselson 与 Stahl 用氮同位素证明了这一点。
Transcription copies a gene into messenger RNA in the nucleus, catalysed by RNA polymerase. In translation the mRNA binds to a ribosome, transfer RNA molecules carry specific amino acids and their anticodons pair with mRNA codons, and peptide bonds form between adjacent amino acids. The genetic code is a triplet code, degenerate and non-overlapping. A mutation is a change in the base sequence; substitution may change one amino acid, while insertion or deletion causes a frameshift and usually a non-functional protein.
转录在细胞核中把基因拷贝成信使 RNA，由 RNA 聚合酶催化。翻译时 mRNA 结合核糖体，转运 RNA 携带特定氨基酸，其反密码子与 mRNA 密码子配对，相邻氨基酸之间形成肽键。遗传密码是三联体密码，具有简并性且不重叠。突变是碱基序列的改变；替换可能只改变一个氨基酸，而插入或缺失会造成移码，通常产生无功能蛋白。
8. Genetics, Mutation and Inheritance | 遗传、突变与遗传学
Genes exist in alternative forms called alleles. A genotype is the combination of alleles an organism possesses, while a phenotype is the observable characteristic, resulting from the interaction of genotype and environment. Individuals with two identical alleles are homozygous and individuals with two different alleles are heterozygous. In a monohybrid cross between two heterozygotes the expected phenotypic ratio is 3:1.
基因以不同形式存在，称为等位基因。基因型是生物体所拥有的等位基因组合，表型是可观察的性状，由基因型与环境相互作用决定。两个等位基因相同的个体为纯合子，不同的为杂合子。两个杂合子之间的单基因杂交，预期表现型比例为 3:1。
Codominance occurs when both alleles are expressed in the phenotype, as in the AB blood group, while incomplete dominance produces an intermediate phenotype. Sex linkage occurs when a gene is carried on the X chromosome; because males have only one X chromosome they are more likely to express recessive sex-linked disorders such as haemophilia and red-green colour blindness. Multiple alleles and polygenic inheritance give continuous variation.
共显性是两个等位基因都在表现型中表达，如 AB 血型；不完全显性则产生中间表现型。伴性遗传指基因位于 X 染色体上；由于男性只有一条 X 染色体，因此更易表现血友病和红绿色盲等隐性伴性遗传病。复等位基因和多基因遗传产生连续变异。
Chi-squared tests are used to compare observed and expected results and to decide whether differences are due to chance. The formula is:
卡方检验用于比较观察值与预期值，判断差异是否由随机因素造成。公式为：
χ² = Σ (O − E)² / E
greater than the critical value, the null hypothesis is rejected and the difference is significant. For the Hardy-Weinberg principle, allele frequencies are described by:
你还需要记住自由度等于类别数减一，以及 p = 0.05 时的临界值。若计算值大于临界值，则拒绝零假设，差异显著。哈迪-温伯格原理中，等位基因频率由下式描述：
p² + 2pq + q² = 1
where p is the frequency of the dominant allele and q the frequency of the recessive allele. Mutation, meiosis, random fertilisation and natural selection are the main causes of changes in allele frequency in a population.
其中 p 为显性等位基因频率，q 为隐性等位基因频率。突变、减数分裂、随机受精和自然选择是种群等位基因频率改变的主要原因。
9. Gas Exchange and Transport | 气体交换与运输
The rate of diffusion across a gas exchange surface depends on Fick s law: rate is proportional to surface area multiplied by the concentration gradient, divided by the diffusion distance. Efficient exchange surfaces therefore have a large surface area, a thin barrier, a steep concentration gradient maintained by ventilation and blood flow, and they are often moist.
气体交换表面的扩散速率取决于 Fick 定律：速率与表面积乘以浓度梯度成正比，与扩散距离成反比。因此高效交换表面具有面积大、屏障薄、由通气和血流维持的陡浓度梯度等特点，并且通常保持湿润。
Mammals use alveoli, insects use tracheae and spiracles, and fish use gills with a counter-current flow system in which water and blood flow in opposite directions so that a diffusion gradient is maintained along the whole length of the gill plate. Plants exchange gases through stomata, which are opened and closed by guard cells, and they balance gas exchange against water loss.
哺乳动物使用肺泡，昆虫使用气管与气门，鱼类使用鳃并采用逆流交换系统：水流与血流方向相反，因此沿整个鳃板长度都能维持扩散梯度。植物通过由保卫细胞控制开闭的气孔进行气体交换，并在气体交换与失水之间取得平衡。
Haemoglobin is a quaternary protein with four polypeptide chains, each with an iron-containing haem group that binds one oxygen molecule. The oxyhaemoglobin dissociation curve is S-shaped because binding of the first oxygen changes the shape of the molecule and increases its affinity for the next oxygen, an effect called cooperative binding. At high partial pressure of carbon dioxide the curve shifts to the right, so oxygen is released more readily to respiring tissues; this is the Bohr effect. Carbon dioxide is transported mainly as hydrogencarbonate ions, some bound to haemoglobin as carbamino compounds, and a small amount dissolved in plasma.
血红蛋白是四级结构蛋白，含四条多肽链，每条链上的含铁血红素基团可结合一分子氧。氧合血红蛋白解离曲线呈 S 形，因为第一个氧的结合会改变分子形状并提高对后续氧的亲和力，这种效应称为协同结合。二氧化碳分压高时曲线右移，氧更易释放给正在呼吸的组织，这就是玻尔效应。二氧化碳主要以碳酸氢根离子形式运输，部分与血红蛋白结合形成氨基甲酰血红蛋白，少量溶解于血浆中。
10. Respiration and Photosynthesis | 呼吸作用与光合作用
Aerobic respiration releases energy in four stages. Glycolysis occurs in the cytoplasm, where glucose is phosphorylated and split into two triose phosphate molecules, producing two molecules of pyruvate, two of reduced NAD and a net gain of two ATP. The link reaction converts pyruvate into acetyl coenzyme A and releases carbon dioxide in the mitochondrial matrix.
有氧呼吸分四个阶段释放能量。糖酵解在细胞质中进行：葡萄糖被磷酸化并裂解为两分子三碳糖磷酸，生成两分子丙酮酸、两分子还原型 NAD，并净得两分子 ATP。连接反应在
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